The present application claims priority under 35 U.S.C. xc2xa7 119 of Austrian Patent Application No. 1232/1999, filed on Jul. 15, 1999, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to austenitic, paramagnetic and corrosion-resistant materials, particularly in media with high chloride concentrations, and materials having high strength, yield strength, and ductility. The invention further relates to processes for producing such materials and methods of using such materials.
2. Discussion of Background Information
High-strength materials that are paramagnetic, corrosion-resistant and, for economic reasons, essentially consist of alloys of chromium, manganese, and iron are used for manufacturing chemical apparatus, in devices for producing electrical energy, and in particular for components, devices and equipment in oil field technology. Increasingly high demands are being placed on the chemical corrosion properties as well as the mechanical characteristics of materials used in this manner.
In essentially all of the applications named above, it is indispensable for the behavior of the material to be completely homogeneous, highly amagnetic, or paramagnetic. For example, in cap rings of generators with high yield strength and ductility, a possibly low-level ferromagnetic behavior must be excluded with utmost certainty, including in parts of the material. For measurements during drilling, in particular exploration wells in crude oil or natural gas fields, drill stems made of materials with magnetic permeability values below about 1.02 or possibly less than 1.018 are necessary in order to be able to follow the exact position of the bore hole and to ascertain and correct deviations from its projected course.
It is furthermore necessary for devices in oil field technology and drill stem components to have high mechanical strength, in particular a high 0.2% yield strength in order to achieve machinery and plant engineering advantages and a high degree of operational reliability. In many cases, high fatigue strength under reversed stresses is just as important because, during rotation of a part and/or drill stems, pulsating or alternating stresses may be present.
Finally, the corrosion behavior of the material in aqueous or oily media, in particular media having high chloride concentrations, is critically important.
As a result of the demands of recent developments in plants and deep drilling technology, increasingly strict criteria are being placed on materials in terms of the combination of paramagnetic behavior, high yield strength, as well as strength, resistance to chloride-induced stress corrosion, pitting corrosion (pitting) and crevice corrosion.
Some materials made from Crxe2x80x94Mnxe2x80x94Fe alloys are known which, with respect to their mechanical characteristics and corrosion behavior, completely fulfill these requirements, but whose magnetic permeability values prevent their use in parts used in connection with magnetic measurements and, for example, exclude their use for drill stems. On the other hand, available amagnetic materials with good strength characteristics cannot resist attacks by corrosion and, for the most part, paramagnetic parts with high corrosion resistance often do not have the necessary high mechanical values.
It is known to use nitrogen content to improve mechanical and chemical corrosion properties of substantially Crxe2x80x94Mnxe2x80x94Fe alloys; however, expensive metallurgic processes operating at elevated pressure are necessary therefor.
For economic reasons, Crxe2x80x94Mnxe2x80x94Fe alloys have been developed that can be produced without pressurized smelting or similar casting processes, i.e., at atmospheric pressure (WO 98/48070), in which a desired characteristic profile of the material is to be achieved using alloying technology. For the purpose of improving corrosion resistance, these alloys have a molybdenum content of over 2% which results in advantages, in particular in pitting and crevice corrosion behavior. However, molybdenum, like chromium, is a ferrite former and can lead to unfavorable magnetic characteristics in the material in segregation areas. While increased nickel contents stabilize the austenite, possibly in conjunction with increased copper concentrations, they may have a detrimental effect on the mechanical characteristics and also intensify crack initiation.
According to PCT/US91/02490, an attempt is made to use a balanced concentration of alloy elements to create an austenitic, antimagnetic, rust-proof steel alloy that, during hot working, and has a beneficial combination of characteristics without further tempering.
A process has been suggested (EP-0207068 B1) for improving, in particular, mechanical characteristics of amagnetic drill string parts in which a material is subjected to a hot and a cold forming, with the cold forming taking place at a temperature between 100xc2x0 C. and 700xc2x0 C. and a degree or deformation of at least 5%.
The invention provides a material, process of making and methods of use.
In an aspect of the invention, a material is provided that is paramagnetic, corrosion-resistant, including particularly in media having high chloride concentrations, and having high yield strength, strength, and ductility, the material comprising carbon, silicon, chromium, manganese, nitrogen, and optionally, nickel, molybdenum, copper, boron, carbide-forming elements (e.g. group 4 and 5 elements), and the balance can include iron, and possibly smelting-associated tramp elements, and impurities. The material is preferably substantially completely austenitic.
Thus, in one aspect, the present invention provides an austenitic, paramagnetic material with good corrosion resistance, in particular in media with high chloride concentrations, high yield strength, strength, and ductility, comprising (in wt-% based on total material weight): up to about 0.1 carbon; from about 0.21 to about 0.6 silicon; greater than about 20 to less than about 30 manganese; greater than about 0.6 to less than about 1.4 nitrogen; from about 17 to about 24 chromium; up to about 2.5 nickel; up to about 1.9 molybdenum; up to about 0.3 copper; up to about 0.002 boron; up to about 0.8 of carbide-forming elements; the balance including iron; and substantially no ferrite content. Preferably, the material is hot-formed to a degree of deformation of at least about 3.5 times and is further formed (i.e., cold-formed) below the deposit temperature of nitrides as well as associated phases, but at elevated temperature, e.g., greater than about 350xc2x0 C.
The material more preferably comprises: less than about 0.06 wt-% carbon; less than about 0.49 wt-% silicon; from about 19 to about 22 wt-% chromium; from about 21.5 to about 29.5 wt-% manganese; from about 0.64 to about 1.3 wt-% nitrogen; from about 0.21 to about 0.96 wt-% nickel; from about 0.28 to about 1.5 wt-% molybdenum.
Preferred embodiments include those materials exhibiting relative magnetic permeability of less than about 1.05, especially less than about 1.016; yield strength RP0.2 of more than about 700 N/mm2 at room temperature; notch impact strength at the same temperature of over about 52 J; FATT of less than about xe2x88x9225xc2x0 C.; fatigue strength under reversed stresses greater than about xc2x1400 N/mm2 at N=107 load alternation; pitting corrosion potential in neutral solutions at room temperature of greater than about 700 mVH/1000ppm chlorides; pitting corrosion potential in neutral solutions at room temperature of greater than about 200 mVH/80000ppm chlorides; grain structure quality grade of DUAL or better in the oxalic acid test according to ASTM-A262.
The material of the invention can be very beneficially used, for example, in connection with oil field technology and equipment, such as for bore rods and drilling string components as well as for precision-forged components, and for high strength attachment and connection elements.
In another aspect, the invention provides a process utilizing novel alloying technology that includes a deformation and synergistically results in production of a ferrite-free material that is paramagnetic with greater reliability and reproducibility, is corrosion-resistant, particularly in media with high chloride concentrations, and has high yield strength, strength, and ductility.
For example, in an aspect, the present invention provides a process of producing a material from an alloy, the material preferably comprising (in terms of wt-% based on total material weight) up to about 0.1 carbon; about 0.21 to about 0.6 silicon; about 17 to about 24 chromium; manganese; nitrogen; optionally up to about 2.5 nickel; optionally up to about 1.9 molybdenum; optionally up to about 0.3 copper; optionally up to about 0.002 boron; and optionally up to about 0.8 of at least one carbide-forming elements, e.g. from groups 4 and 5 of the periodic system. The balance can include iron, smelting-associated tramp elements, and impurities. Manganese is preferably incorporated in the material at from greater than about 20% to less than about 30% by weight. Nitrogen is preferably incorporated at from greater than about 0.6% to less than about 1.4% by weight.
In another aspect of the invention, a process is provided, wherein an alloy is smelted with introduction of manganese and nitrogen, allowed to solidify under atmospheric pressure to produce an ingot or casting, and the ingot or casting formed thereby, is subjected to a hot forming or forging and subsequently actively cooled at an increased rate, whereupon a further forming (i.e., cold-forming) of the piece occurs at a lower temperature, and then the formed part is allowed to cool to room temperature. The ingot or casting can be produced by an electroslag remelting process.
In a preferred embodiment the ingot or casting is subjected to an intermediate annealing after the hot-forming at temperature at least about 850xc2x0 C. and subsequently to a cooling at an increased rate.
Preferably, the hot-forming introduces a degree of deformation of at least about 3.5 times and the further forming is conducted to a deformation of less than about 35%, more preferably about 5% to about 20%. The further forming is preferably carried out at temperature in the range of about 400 to 500xc2x0 C.
Preferably, the cooling at an increased rate is an intensified cooling to and maintenance at a temperature below about 600xc2x0 C. and, after the temperature has equalized, over its cross section, is conducted to the further forming.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure.